Observe that Trump’s debate success throughout the Republican Party primary contest was mostly against a crowded field — and never once one-on-one.

Moreover, months of polling have demonstrated repeatedly that Trump gains ground against Mrs. Clinton when pollsters offer voters the additional choices they’ll see on their ballots: namely, the Libertarian and Green Party candidates, Johnson and Stein. Johnson is on all 50 state ballots and Stein will be on in forty-four.

“Overall, including third-party candidates takes about 1 percentage point away from Clinton’s margin, on average,” concluded a polling analysis by Harry Enten at FiveThirtyEight. Granted, one percent isn’t much . . . but just possibly the margin of difference, i.e. the difference between winning and losing.

Neither Johnson nor Stein met the 15 percent threshold set by the hostile debate commission. No third-party or independent candidate has ever jumped over that moon. Yet, though Johnson’s polling average in the five national voter-surveys picked by the debate commission was only 8.4 percent, he has polled 15 percent in 15 different states and at least double-digits in 42 states.

Note: presidential elections are determined state by state, not by the popular vote nationally.

In several swing states, Johnson will easily be the margin of difference. And a recent Washington Post poll showed former Gov. Johnson at 25 percent in his home state of New Mexico, just two points shy of Mr. Trump. Meanwhile, Johnson is at 23 percent in Utah, four points below Clinton.

Now, however, denied a chance to debate before voters, support for Johnson is likely to fade. As many as 100 million Americans will watch this first debate and be not so subtly nudged to view Johnson and Stein as the not-ready-for-prime-time players— or forget they’re even running. Which will cause many to switch to Trump or Clinton.

But which one?

As mentioned, the polling indicates Clinton (not Trump) will be helped slightly by pushing third-party candidates out of sight. But let me suggest the advantage to Mrs. Clinton will be much more than slight.

Consider, first, how important the youth vote, comprising roughly one-fifth of the total vote, is to the Democratic Party. The headline to a Politico postmortem on the 2012 presidential race said it succinctly: “Youth vote was decisive.”

Had Republicans split the youth vote, “Mitt Romney would have cruised to the White House.” Instead, a study conducted by the Center for Research and Information on Civic Learning and Engagement at Tufts University found that Obama’s victory was made possible only by the huge margins he rolled up among 18 to 29-year-olds, especially in the crucial swing states of Ohio (27 points), Florida (34 points), Virginia (25 points) and Pennsylvania (28 points).

Then, let’s also remember how badly Democrat Hillary is doing among millennials.

“Not that they were ever so sweet on [Hillary Clinton] to begin with, at least relative to how they swooned over other Democrats,” she added.

More importantly, Rampell explained: “In most of these polls, the young supporters ditching Clinton seem to be shifting not to Trump but to third-party candidates, particularly Libertarian Gary Johnson. The Michigan poll has Johnson tied with Trump; the national Quinnipiac poll actually has Johnson slightly ahead of Trump among under-35 voters.”

“If Hillary Clinton loses the election to Donald Trump,” Tina Nguyen bemoaned at Vanity Fair, “it will be because young people didn’t vote for her. That’s not to say that they like Trump — on the contrary, polls show millennials dislike the Republican nominee by historic proportions.”

And that’s the rub.

If millennials decide Gov. Gary Johnson is merely a protest vote and that they, therefore, must decide between Hillary or Trump, they are likely to opt for Hillary Clinton — as the Post’s Rampell put it, “even if they do so grudgingly.”

An August Investor’s Business Daily/TIPP poll revealed that 65 percent of folks under 25 years of age viewed Mrs. Clinton unfavorably, while Mr. Trump’s unfavorable score was an incredible 88 percent. That poll found the Libertarian Party standard-bearer leading nationally among these young folks, with 35 percent support. At 30 percent, Clinton took second, and Dr. Jill Stein edged out The Donald with 14 percent to his 12 percent support.

At Red Alert Politics, editor Ron Meyer wrote, “[T]he poll shows Johnson could still win over more young Democrats. A shocking 75 percent of young Clinton supporters say they only support her ‘moderately,’ while 16 percent support her ‘strongly.’ Johnson’s liberal social positions and his non-interventionist foreign policy could help him win former supporters of Bernie Sanders.”

Unfortunately, with Johnson’s absence from the debate, the pivotal event of the campaign, those wavering “I’m (only reluctantly) with Her” young whippersnappers are left with no alternative but to stick with Hillary.

Mr. Trump and his supporters probably relish going face-to-face against Mrs. Clinton in tomorrow’s debate. But when the votes are counted this November, they may well discover that keeping Libertarian Gary Johnson off the debate stage turns out to be — in the Trump vernacular — “a disaster.”